Existing communications systems provide several methods to transmit information. A system, intended to separate channels (subscribers) by frequency, within each subscriber is assigned a unique bandwidth is known as Frequency Division Multiple Access (FDMA). In such a design, the bandwidths should not overlap one another, otherwise they would interfere with one another.
A TDMA (Time Division Multiple Access) system tries to time-share subscribers in an in-sync state while such subscribers work in the same bandwidth. It is crucial in a TDMA system that the time-sharing is separated for each subscriber and should not overlap the existing bandwidths of other subscribers.
FDMA and TDMA systems have the challenge of trying to protect the transmitted data from external interferences. To enable the subscriber to reproduce the useful information transmitted with sufficient accuracy, the level of useful signals must typically be a number of times, such as 4 or 5 times, greater than the level of interference.
Another existing technology includes a multiple access system with code division of subscribers CDMA (Code Division Multiple Access) that uses the spread-spectrum signals. This enables plural subscribers to communicate in the same bandwidth at the same time in case the coding is orthogonal, i.e. the cross correlation is equal to zero.
The signal detection is achieved using the correlation method and the criterion for signal detection is the cross correlation. Hence, these systems have the problem of trying to separate the useful signal from interferences that are non-orthogonal with respect to the useful signal. In other words, due to nonlinear detection of the signal, these existing systems are not well protected from orthogonal interference when the interference level significantly surpass the level of the useful signal.
The ability to send and receive information, maintain its accuracy and security in the presence of orthogonal and non-orthogonal interference are significant challenges. The challenges are further compounded when interference level is significantly more powerful or greater than the useful information signal.